1. Field of the Invention
The present invention generally relates to gain control of a variable-gain amplifier and, in particular, to a gain control method and apparatus suitable for a system necessitating fine gain control such as a DS-CDMA (Direct Sequence-Code Division Multiple Access) system.
2. Description of the Related Art
In a DS-CDMA system, a plurality of channels are assigned to a single frequency band, and each user's PN (pseudorandom noise) sequence is generated by a unique code to allow receivers to distinguish among different user's signals. That is, the receiver performs reverse-spreading or despreading of the received signal by multiplying it by a replica of the PN sequence used at the transmitter. Since the receiver uses a PN code unique to the desired user signal, the signals coded with other users' codes simply appear as noise, resulting in deteriorated quality of communications.
Therefore, it is important that the power of each user's transmitter is controlled by the central base station so that the same power is received from every terminal and the power level is constant with time. In other words, the transmission power of each terminal must be precisely controlled in any DS-CDMA system serving mobile users. According to Interim Standard 95 (IS 95) released by TIA (Telecommunications Industry Association) in North America for Code Division Multiple Access (CDMA), transmission power tolerance is specified at .+-.0.5 dB. Further, the receiver performs reverse-spreading of the received signal to distinguish among different users' signals. Therefore, fine gain control is also necessary for the receiver of each terminal to perform linear signal processing.
The transmission power control and the gain control as described above are performed by an automatic gain control (AGC) circuit using a variable-gain amplifier. More specifically, the output level of the variable-gain amplifier is compared with a target level, and the gain of the variable-gain amplifier is controlled so as to reduce the difference between the output level and the target level according to the comparison result.
To achieve the precise AGC characteristic, the gain of the variable-gain amplifier should be linearly changed according to a gain control signal S.sub.D over a wide dynamic range. However, in general, a variable-gain amplifier does not have the linear gain control characteristic over its whole dynamic range but a non-linear gain control characteristic as shown by a characteristic curve 10 in FIG. 1A. Therefore, it is necessary to correct the gain control signal S.sub.D applied to the variable-gain amplifier so as to provide the linear gain control characteristic.
According to a conventional AGC circuit, a correction table is previously stored onto a memory such as ROM, and the gain control signal S.sub.D to be applied to the variable-gain amplifier is corrected using the correction table. More specifically, as shown in FIG. 1A, a plurality of discrete points (in this figure, gain values G.sub.1 -G.sub.9 and gain control values C.sub.1 -C.sub.9) are previously sampled from the characteristic curve 10 of the variable-gain amplifier over the gain control range, and a set of discrete data showing the relationship between the discrete gain values G.sub.1 -G.sub.9 and gain control values C.sub.1 -C.sub.9 of the gain control signal S.sub.D is stored in the memory. Such discrete data reduce the amount of data stored in the memory. By the linear interpolation using the discrete data of the correction table, the gain control signal S.sub.D is corrected to provide the linear gain control characteristic to the variable-gain amplifier.
Such a control method using a correction table as mentioned above has been disclosed in Japanese Patent Laid-open No. 63-167557. Although the control circuit is included in a semiconductor laser driver, the output power of the laser is automatically controlled by a feedback loop using the correction table.
However, the conventional control method and AGC circuit cannot provide precise correction of the gain control signal S.sub.D to be applied to the variable-gain amplifier. As described above, the gain control range of the variable-gain amplifier is equally divided to obtain the discrete gain values G.sub.1 -G.sub.9 as shown in FIG. 1A. And the gain control values between the discrete values of the gain control signal S.sub.D can be obtained by linear interpolation from the correction table. Therefore, in cases where the characteristic curve 10 has a sharp curvature, the corrected gain control signal derives from an ideal gain control signal at the location of that sharp curvature.
More specifically, when the characteristic curve 10 has a sharp curvature (for example, between the gain control values C.sub.1 and C.sub.2, C.sub.7 and C.sub.8, or C.sub.8 and C.sub.9 in FIG. 1A), as shown in FIG. 1B, a corrected output characteristic curve 11 of the variable-gain amplifier deviates from an actual output characteristic curve 12 between sample positions S.sub.1 and S.sub.2, S.sub.7 and S.sub.8, or S.sub.8 and S.sub.9. Such a deviation causes the power control in the DS-CDMA system to be deteriorated, resulting in reduced quality of communication.